You can water any time of day. The key is to make sure that the foliage remains dry and the water goes in the ground rather than running off. I like to gently and quickly water the soil to get it wet, come back to it in a little bit and water it a little more, and then make a heavy pass over the area after this has had a chance to sink in. This will in effect create a basin to hold the water in the soil, and ensure that your water is going into the root zone rather than running off. If your water is running off you are not watering.

What kind of potting soil are you using?

The idea that watering the foliage in full sun or heat will "burn" the leaves is completely bogus. Water droplets do not refract light like a lens to burn holes in leaves. Patently untrue. In fact, many plants will very much appreciate a cooling shower in the middle of the day. Not only does the water carry away heat simply by being in contact with the plant, but it further cools the leaves as it evaporates.

The reason foliar watering is not recommended is that if it does not evaporate fast enough, it can promote diseases (both fungal and bacterial). At the time most people can water, early morning or late evening, this is a major problem. If the leaves stay wet all night for days on end, fungal disease is very likely in susceptible plants.

On the other hand, watering at the very peak of heat and sunshine can significantly improve performance and *prevent* disease. Fungal spores will begin to germinate because of the moisture, and this leaves them in a vulnerable state. When the sun and heat quickly evaporate away the water, the half-germinated spores desiccate and die. This is true for many, but certainly not all, important diseases. One confirmed example is blackspot on Roses, which also greatly benefit from a cooling midday shower.

When you're container gardening in the Deep South, you have to make arrangements for the oppressive heat and sun. Larger containers help. So does staying away from thin plastic containers or metal containers. Large white, heavily insulated containers or large unglazed terracotta work very well.

I use Hortus's doubling back technique, as well as drench my containers until water is freely flowing from all of the drainage holes. Light shade for the plant can help, as well as heavy shade or complete blockage of light for the container and soil. People use plywood boxes, makeshift "tree-skirts", and all sorts of other methods to do this.

I always just put my water nozzle set to the "shower" setting and put it directly touching my mulch so that no spray ever touches the leaves, makes no difference night or day.

But in the heat of the day is actually a good time to give them a nice cooling drink of water.

In the heat of the day, I just make sure I run my hose until I feel cool water coming out first. The coiled up hose holds a fair amount of water, and it heats up really hot... The plants won't really appreciate being watered with 100+ degree water...

I have been well educated for sure. Thank you all for your great advice and knowledge.

Thanks greentiger87. Good information about fungus disease.
I've been watering my veggie leaves in the sun for awhile now in order to cool them down on hot days. Never once saw any burn on the leaves.

I'll have to try it now on roses and see if it does reduce their blackspot.

bspatial5,
Faster draining mixes do wilt faster and depending on the mix may always need an afternoon drink to prevent wilting.

Middle of a hot day is not always the best time to judge the thirst of a plant. Many plants that don't really need more water on their roots can wilt a bit on a hot afternoon in the ground, but especially in a pot. Potting soil is more likely to hold excess water than dry out faster if it is comprised of mostly peat. However, if it truly dries out, it's very difficult to get it to absorb water again. Even if afternoon is when you have time to water, try checking the plants in the morning and evening. If they are wilty when it's not hot, they truly need water. If you're still not sure, stick your finger down in the dirt to see if it's truly dry under the surface.

Figs are just coming ripe the past few days here. What kind do you have? Turkey fig? What kind of containers? If they are black, that can really heat up the roots.

The whole key to growing tomatoes in containers is to use LARGE containers.
It helps keep the roots cooler, roots have more water available to them for a longer period of time.
Roots grow larger so can support a larger plant and more fruit.
In a properly set up large pot and good draining potting mix it is impossible to over water.

My plants have been growing since April, we have had several days between 94-98 degrees and this Friday is going to break 100 for sure and my plants have never once wilted or drooped at all. They look the same all day long because they have a large root system.

A plant is sort of like a plumbing system, the roots supply the "water pressure" in the system/plant.
Larger and healthy roots suck up more water and nutrients and keep that water pressure up, so plants never wilt/droop.

Mine never wilt, even over 90F, unless they have only been recently transplanted. Once the roots have established well in a well drained media, wilt does not occur for me.

My wife still uses peat based soil sometimes and occasionally experiences midday wilt in that soil. I suspect it's because the roots are much thinner in that medium then the bark based medium I use that creates large white "herringbone" roots very similar to what I get in hydroponics.

If you water in the evening, as I often do, it's best to try to keep water off the foliage. Fungal spores need an incubation period, during which they are constantly supplied with moisture. When watering at midday, foliage dries more rapidly than if you water just before dark. Often, foliage will remain wet for the entire night, which supports proliferation of the fungaluglies.

Al

Nunyabiz1 and Capoman,
How large are your containers? What time of day do you water?
In your climate do you get intermittent cloud cover?

I'm focusing on "overcoming wilt" this summer and looking to learn as much as I can about it. My deck containers only get sun from 10am to 2:30pm, during the heat, so wilt really inhibits plant growth.

My pots are at least 15 gallons, they hold about 60 quarts of potting mix.

No particular time of day really, sometimes mornings, sometimes afternoons, just when ever the mood strikes me or dependent on weather.

My Bush tomatoes are tied to my deck railing so they stay in one spot and get about 6+ hours of sun a day.
My Dwarfs are movable still and will probably remain so, I have them on one side of the deck in the morning and the other side in the afternoon, they get about 9 hours of sun a day.
Just use large pots, at least 15 Gallon if they are Determinate/Bush/Dwarfs and big as you can manage if they are Indeterminate. 65 Gallon Smart Pots would be ideal, but I would say at least 25-30 gallon.
Use a good potting mix that drains well.

Once the plant is established with a good root system you should not have any wilt as long as you water them everyday.

Nunyabiz1, while I'm still salivating over those pictures of your garden, I think you deeply underestimate exactly how intense the conditions are in the Deep South during the summer. The last few days have seen temperatures 6-8 hours of temperatures in the mid 100's and humidity levels above 70%. And that's just the air temperature.. the real bear is the intensity of the solar radiation hitting the leaves, the soil surface, and the outside of the pot.

I've used 30 and 40 gallon smart pots, or similar sized black nursery pots, and still had serious issues with heat build up in the root zone. Does the larger mass of soil and the larger mass of water it can hold help? Yes. Is it always enough to prevent plants from wilting? No. Does it prevent the reduction in potential growth and yield caused by overheated roots? No.

Using a soil thermometer, I've found that I can significantly reduce the problem with little effort.

Nunyabiz1,

Do you then use 15 gallon smart pots? It wasn't totally clear.

I do think smartpots help reduce wilt. It just set up a 15 gallon one.
Whenever I grow seedlings in soil blocks or even transplant store bought seedlings into net pots, they never wilt. Now these are small 5" pots, but still even in 100 degrees the seedlings look vibrant.

My peppers in a 15 gallon tub with a fast draining mix, wilt very fast, even if they've been watered in the morning.

Might just be the difference in cloud cover and temperature in our different zones.

I WANT to use 15+ gallon smart pots and will next year.
I just learned of them.
My pots are regular 15+ gallon plastic pots.

Greentiger: Maybe in that case use a sun shade to help cool the plants and water in the heat of the day to keep the roots cool.

Umm... are you being ironic or something?

Ironic about what?

Using shade or watering in the heat of the day are exactly the two major suggestions I made to the OP. 2nd reply down. That's what I meant by being able significantly reduce the problem with a little forethought. I've also mulched with perlite, made wood boxes that cover everything but the plant, spraypainted black nursery pots white... etc.

Ack, I didn't even read that.
In that case I agree with everything you stated.

In fact "mulching with perlite" gives me an idea I might try for next year.
I was going to just use that red plastic sheet as mulch next year instead of cypress mulch, but I think a nice 2" layer of large perlite under that plastic sheet would be a good idea.

It is 108 degrees right now with a heat index of 114.

Plants have been in the sun all day since 9am with zero drooping of any kind. In fact they look amazingly good.
Have watered them lightly 3x though just to cool the roots.

Tomorrow is supposed to be hotter.

Yes. I've noticed that if I water every 2 hours on a hot day, the fast mix tub plants will not wilt. I believe though it is because water is being poured all over the feeder root hairs, so it can wick up more.

Plants in compressed peat soil blocks will not wilt on 100 degree days either. I've measured the soil temp, and it is the same as the soil temp in tubs where the plants wilt (unless frequently watered). So I've concluded that the wilt isn't necessarily or primarily caused by the soil heat.

Believe it is a combination of wicking capacity and air exchange. The peat soil blocks don't have the porosity the fast draining mix has, but maybe the high evaporation in heat causes enough of an air exchange in the soil block.